The present invention relates to a flow force compensating method for a flow control valve having a fluid passage structure in which control fluid flows from a valve stem space, via a control orifice, into a valve land space defined between a spool land and a circumferential groove formed in a sleeve, and such a flow control valve of spool type using the same method.
In flow control valves of spool type, there has been proposed a method for supporting a valve spool by hydrostatic bearings in order to improve the sliding property of the spool. For example, Japanese Patent Application No. 8-182016 (1996) according to the Applicant describes a hydraulic electromagnetic proportional control valve using hydrostatic bearings.
FIG. 13 is a sectional view showing an example of a flow control valve using such hydrostatic bearings. In FIG. 13, a spool 1 is slidably mounted within a sleeve S and hydrostatic bearings 19 are provided on both ends of the spool. By supplying pressurized fluid from a supply port P to the hydrostatic bearings 19, the spool is supported within the sleeve without contacting with the sleeve. The spool 1 can be shifted to the left or the right from a neutral position as shown by a spool drive means D such as a solenoid, so that a valve opening is adjusted by the position of lands 2, 2a, 2b, thereby controlling flow rates of fluid flowing between ports P, 17 and 18.
FIG. 11 shows the spool 1 incorporated in the flow control valve stated above.
In FIG. 11, the spool 1 has lands 2a and 2, 2b having a large diameter at its central portion and both end portions and stems 3, 3a having small diameter between the lands, and the lands 2, 2b are supported by hydraulic bearings. Incidentally, the spool shown in FIG. 11 is a conventional one and is not subjected to flow force compensation which will be described later.
In the conventional flow control valve, however, when the valve is opened to flow control fluid around the spool, a flow force tending to close the valve is generated. FIG. 12 shows a condition wherein the spool is shifted from a neutral position to the right so that the fluid flows from a control port 18 to a return port 17. In FIG. 12, when the fluid flows through a valve stem space 15, different pressure distributions are generated on both end walls defining the valve stem space 15. The difference in pressure distributions creates a so called flow force F acting on the spool 1 which tends to close the valve.
When the flow force thus caused is great, a large driving force is needed to shift the spool from the neutral position to an operating position, and, thus, a drive means having greater output power is needed for driving the spool. If the drive output is insufficient, the spool cannot be shifted, thereby causing a trouble in valve operation.
Further, the flow force sometimes generates self-excited vibration in the spool which deteriorates the valve function.
As one of techniques for compensating such flow force, there has been proposed a method in which a circumferential groove is formed in the spool lands and swirl flow is generated in a valve land space (called as "bucket") defined by a circumferential groove formed in the sleeve and the circumferential groove formed in the spool. In this method, however, the swirl flow is not positively generated and the flow force cannot sufficiently be compensated.
To eliminate the above-mentioned inconvenience, conventionally, there has been proposed an attempt in which desired swirl flow is provided by changing configurations of the circumferential grooves in the sleeve and the spool, i.e., changing a configuration of the bucket. However, in this attempt, the configurations of the circumferential grooves are relateively complicate and troublesome working is needed to machine the circumferential grooves accurately. In particular, since the circumferential groove of the sleeve is formed by cutting an inner peripheral surface of the sleeve, it is difficult to form such a circumferential groove accurately in the sleeve.